Here are a couple of threads from the past:
rain water
acid rain water collection

That last thread has a great link to rain water harvesting from Sue. Awesome information there, though it doesn't look easy or cheap.

One thing I would be concerned about is any pesticides on the roof, such ass moss killer. I know some roofs have that sprayed on when they're new to keep moss from being a problem. I've got some in my garage right now, ready to go on the roof to get rid of some issues I'm having. That might preclude collecting rain water there, with it being a chemical contaminate and not just a particulate contaminate.

Good luck!
Jason